The present invention relates to a highly oriented resin-made reinforcing member and a process for producing the same. More particularly, the present invention relates to a highly oriented resin-made reinforcing material that has high tensile strength properties and a suitable degree of flexibility and which is useful typically as a reinforcing member in an optical fiber cable.
A variety of reinforcing members are employed in order to mechanically protect vulnerable articles by means of supporting and reinforcing them. A typical application of such reinforcing members is in the glass fiber cables used for light transmission purposes.
Glass fibers used for light transmission purposes are brittle and are not easily bendable, so individual optical fibers emerging from the drawing die are usually provided with protective and other outer coatings. The optical fibers having such primary and secondary coatings are then assembled into a cable, with plastic (e.g. polyethylene) coated steel wires and metal pipes being incorporated as tension members so as to render the coated fibers resistant to the forces that will develop during and after installation operations.
An example of such conventional optical fiber cable is shown in cross section in FIG. 10. Being generally referred to as the layer stranded type, the cable shown in FIG. 10 consists of a plurality of coated fibers 2 that are stranded in a layer form around a central tension member 1 and are provided with a tubular coating 3. The central tension member 1 is usually made of a steel wire coated with a plastic material such as polyethylene, and the tubular coating 3 is typically formed of a plastic material such as polyethylene.
The metallic tension member has the advantage of imparting satisfactory tensile strength to the cable but, on the other hand, the cable becomes susceptible to electrical shock and electromagnetic induction. In addition, the cable with the metallic tension member is heavy and is not easy to handle during installation operations. Furthermore, some provisions must be made for protecting the cable from corrosion and other attacks that may occur during storage and service periods. These disadvantages are sometimes serious enough to reduce the inherent advantages of the optical fiber such as lightweightness, immunity to electromagnetic induction and cross talk, low transmission loss and high transmission capacity. It is therefore strongly desired to develop non-metallic tension members that are lightweight and exhibit greater durability under corrosive and other hostile conditions.
The tubular coating on the optical fiber cable is customarily made of easily extrusion-coatable resins such as polyamide and polypropylene, but most of them have low Young's moduli (.ltoreq.ca. 100 kg/mm.sup.2) and high thermal expansion coefficients (ca. 10.sup.-4), which differ considerably from the respective values of the glass used as the principal component of the optical fiber (7,000 kg/mm.sup.2 and ca. 10.sup.-7). Therefore, a non-metallic optical fiber cable in which both the central tension member and the tubular coating are made of a conventional, easily extrusion-coatable resin material will experience a significant increase in transmission loss as a result of temperature changes or if it is subjected to tensile force.
Under low temperature conditions, the central tension member and the tubular coating will contract to a greater extent than the fiber itself and the resulting stress will produce bends and lateral compression in the fiber. These effects are major factors in the occurrence of transmission loss (microbending loss) and/or dispersion. If, on the other hand, the central tension member and the tubular coating are stretched under high temperatures or upon application of great tensile force, the fibers are also affected and show stress distortion, which is another cause of transmission loss and dispersion. Therefore, the optical fiber cable in which the central tension member and tubular coating are solely made of the conventional extrusion-coatable resin will inevitably experience a noticeable increase in transmission loss as a result of temperature changes or upon application of high tensile force.
With a view to improving the physical properties of the resin-made members used in optical fiber cables, attempts are being made to introduce high molecular orientation in the resinous members. One method that has been proposed as a result of these efforts involves drawing polymers such as ultrahigh molecular weight, polyethylene, polyethylene terephthalate and polyoxymethylene so as to introduce molecular orientation in the polymers in the longitudinal direction, or in the direction parallel to the axis of the cable. By causing such molecular orientation in the resinous member in the cable's axial direction, a Young's modulus that is at least twice the value for the undrawn resin and a linear expansion coefficient (ie, coefficient of thermal expansion in the direction of orientation) that is no more than half the value for the undrawn resin can be attained. The drawn resinous member therefore exhibits a commercially satisfactory tensile strength, a thermal expansion coefficient that is not different from that of the fiber by a factor greater than 100, and even a reasonable degree of resistance to lateral compression. These features combine to provide the possibility of solving the problem of microbending associated with lateral compression.
However, an oriented resin exhibiting satisfactory strength in the direction of orientation has a very low strength in the direction orthogonal thereto. This insufficiency of strength in the transversal direction may be so great that the resin can be easily torn apart by hand along the axis of orientation. A member made of such oriented resin will be affected and torn apart fairly easily if it is subjected to external forces. It may be ruptured under high lateral compression, and may readily buckle if it is bent.
Several methods have been proposed for solving these problems and, as shown in Unexamined Published Japanese Patent Application No. 105601/1984 with respect to the loose tube-type, coated optical fiber, the tubular coating made of a longitudinally oriented resin is coated with a layer of a conventional, easily extrusion-coatable resin material in a substantial thickness so as to provide the tubular coating with improved resistance to tear and lateral compression. But then, the substantial thickness of the outer coat will impair the flexibility of the fiber and increase the overall diameter and weight of the cable. Because of these obstacles, the use of a tubular coating solely made of an oriented resin involves considerable difficulty and has met with no commercial success.
According to another recently proposed method for introducing molecular orientation, a high molecular weight, liquid-crystal material such as a benzoic acid copolymer that shows crystallinity in a molten state is shaped at high shear rate or drawdown ratio (see Unexamined Published Japanese Patent Application No. 202405/1983). But this method has very limited flexibility in shaping conditions and the physical properties of the shaped polymer are not completely satisfactory for commercial use since it has problems similar to those described in connection with the longitudinally oriented resin.
As shown in detail hereinabove, none of the known highly oriented resin made reinforcing members have sufficient tensile strength to be useful as tension members for incorporation in optical fiber cables.